Aerosols



United States Patent 3,144,385 AEROSOLS Frank Qiifton McGrew, Brandywine Hundred, Del., assignor to E. 1. du Pont de Nemours and Company, Wilmington, Deb, a corporation of Delaware No Drawing. Filed Apr. 7, 1945, Ser. No. 587,218 6 Claims. (Cl. 167-39) This invention relates to improved methods for producing dispersions of finely divided solids in air.

Solid aerosols have previously been prepared by various methods, such as by subjecting finely divided solids to explosive bursts. Aerosols prepared by these methods, however, are not as uniformly dispersed as desired. In many cases the aerosols contain many clusters or flocks which remain undispersed due to the cohesive forces between the individual particles. The particles in such aerosols thus settle rapidly and are, therefore, of little practical value. In other cases, when the finely divided solid is stored for long periods of time the individual particles tend to pack together and when subjected to an explosive burst do not give uniform dispersions of in dividual particles.

This invention has as an object the provision of an improved method for dispersing finely divided solids in air. A further object is to provide a process for preparing improved aerosols. A still further object is the provision of a process for producing dispersions in air of finely divided solids in which the cohesive forces between the individual particles are markedly reduced. A still further object is to provide a process for generating aerosols which are capable of remaining suspended in air for hitherto unattainable long periods of time. Additional objects will become apparent from an examination of the following description and claims.

These and other objects and advantages are accomplished according to the herein described invention which comprises reducing the particle size of a solid material to less than 20 microns weight median diameter, surfacecoating the resultant finely divided solid with a polyhydric alcohol partial ester of an unsubstituted long-chain fatty acid acting as a surface-active agent to reduce cohesion between the particles, and subjecting the resultant surface-treated solid material to a bursting charge.

In a more restricted embodiment this invention comprises subjecting to a bursting charge a closed receptacle containing a solid material which has a particle size of less than 10 microns weight median diameter and which is surface coated with from 0.1% to of its weight of a polyhydric alcohol partial ester of an unsubstituted long chain fatty acid.

One preferred embodiment of this invention comprises reducing the particle size of a protein, e.g., blood albumin, egg albumin, gliadin, zein, casein, or the like, to less than microns weight median diameter, surface-coating the resultant finely divided protein particles with from 0.1% to 2% by weight of diethylene glycol monolaurate and subjecting the resultant surface-treated protein to a bursting charge in the air.

Another preferred embodiment of this invention comprises reducing the particle size of an insecticide, e.g., pyrethrum, rotenone, ferric dimethyl dithiocarbamate, p,p'-dichlorodiphenyltrichloroethane, dodecyl thiocyanate, lead arsenate, calcium arsenate, sulfur, basic copper carbonate, barium fluosilicate, calcium fluosilicate, or the like, to less than 10 microns weight median diameter, surface-coating the resultant finely divided insecticide particles with from 0.1% to 2% by weight of diethylene glycol monolaurate and subjecting the resultant surfacetreated insecticide to a bursting charge in the air.

Various arrangements and selections of equipment for the operation of my process are possible. In a preferred arrangement the particle size of the solid material is reduced to a weight median diameter of less than 10 microns by any method appropriate to the particular material being employed. The particles of the resultant finely divided solid are then surface-coated with from 0.1% to 2% of their weight of a dispersing agent, such as a polyhydric alcohol partial ester of a long chain fatty acid, e.g., diethylene glycol monolaurate, whereby cohesion between the individual particles is reduced. The treated powder is placed in a container and then subjected to the disruptive burst of an explosive or to a burst produced by the rupture of a closed container charged with a compressed gas or low-boiling liquid.

The following examples, in which proportions are in parts by weight unless otherwise specified, are given for illustrative purposes and are not intended to place any restrictions on the herein described invention.

Example 1 A ball mill containing /2-inch steel balls is charged with 7000 grams of commercial egg albumin, 11,000 grams of the organic liquid sold commercially as Atlantic Safety Kleen, and '70 grams of diethylene glycol mono-- laurate. This mixture is ground for two days, the organic medium removed by distillation, and the dry residue disintegrated in a Mikro-Pulverizer until the particles have a weight median diameter of 6.8 microns, at which point 76% (by Weight) have a diameter of less than 10 microns and 30% a diameter of less than 5 microns. A cylindrical cellophane container is charged with 2.83 grams of this surface-treated egg albumin, a number 6 electric blasting cap is placed in an axial position in the container, and the assembly placed in a chamber having a volume of 1000 cu. ft. When the cap is fired, an egg albumin aerosol is obtained. Determination of the mass concentration of solid particles in the aerosol by the method described below indicates that 42%, 11% and 2% of the theoretical amount (the total weight of solid originally used for the generation of the aerosol) of albumin remains suspended in the air during periods of one to five, six to ten and eleven to fifteen minutes, respectively, after the explosion.

The aerosol is generated in a closed chamber approximately 10 ft. x 10 ft. x 10 ft. in dimensions at a point approximately 2 ft. below the geometric center of the chamber. A circulating fan placed directly below the aerosol generating apparatus is run at a rate which circulates 1500 cu. ft. of air per minute to obtain a uniform distribution of the aerosol in the test chamber. This chamber contains a set of nine Pyrex glass tubes filled with Pyrex glass Wool carefully placed to eliminate channels without packing to the point of high resistance, each of which has been dehumidified by drawing through it over an eight minute period 1.4 cu. ft. of air which has been dried by calcium chloride. These tubes are arranged in three sets of three each on a vertical frame which stands two feet away from the center of the floor of the chamber. In each set of three, one tube is placed horizontally at each of three levels /2 ft., 5 ft. and 8 ft.) from the floor of the chamber. Each set of three tubes (A, B and C) is attached to a branching vacuum line of rubber tubing, the single end of which leads through the chamber to an individual stocpcock on a vacuum manifold. The vacuum line leads from the manifold through a dry test meter which is provided with dials for reading gas volumes in decades from 5 of a cubic foot upward. The vacuum source is an air pump with capacity of 3 cu. ft. of air per minute. A differential manometer is attached to the vacuum line side of the meter so that the meter reading can be corrected to atmospheric pressure. Exactly one minute after the generation of the aerosol the circulating fan is stopped and filtration of the aerosol through Set A of the filter tubes is begun. Sampling through Set A is terminated at five minutes and Set B and Set C are used in the same way during the intervals of six to ten minutes, and eleven to fifteen minutes, respectively, after the generation of the aerosol. Gas meter readings are obtained before and after each filter period and manometer readings during each one. The filter tubes are again dehumidified before removal from the filter positions by drawing dry air through all nine tubes at the rate of 1.6 cu. ft. per minute for eight minutes. The tubes are then removed and stoppered, carefully wiped with a clean chamois, and weighed. In calculating the mass concentration of the aerosol from these measurements, the volume of air passed through the filter is first calculated by the formula:

where V is the volume in cubic feet recorded by the meter, B is the barometer reading in mm. of mercury, M is the manometer reading in mm. of mercury, and V is the corrected volume of filtered air in liters. The mass concentration is calculated by dividing the weight gain for each set of filters by the volume of air (V passed through the set. Blank runs made with number 6 blasting caps alone indicated that the explosive itself produces a mass concentration of 7 mmg. per liter which does not change appreciably during fifteen minutes. Consequently, the mass concentration determined for a solid powder dispersed by a blasting cap is corrected by subtracting 7 mmg. per liter from the observed concentration.

Example 11 A cylindrical steel vessel having a volume of about 0.5 cu. in. and fitted with a capillary inlet for gas and a shear disc head is charged with 1.98 grams of surfacetreated egg albumin prepared as in Example I. The opening in the shear disc head is closed by discs of cellulose acetate of aggregate thickness sufiicient to require a pressure of about 700 lbs/sq. in. to cause their rupture. Carbon dioxide gas is admitted through the capillary inlet until the internal pressure ruptures the cellulose acetate discs and disperses the finely divided surface-treated protein in a single disruptive burst into a chamber having a volume of 1000 cu. ft. Determination of the mass concentration of solid particles remaining suspended in the air for one to five minutes, six to ten minutes, and eleven to fifteen minutes after the explosion, in three concordant experiments, indicates that, on an average, 64%, 39% and 27% by Weight of the particles are still suspended in the air during these respective periods of time. The corresponding figures for a control dispersion of untreated egg albumin dispersed by the same method are 29%, 9% and 7%, respectively. The settling rates of the dispersed surface-treated particles, calculated as described below, indicate that they have a particle size of about 13 microns.

The settling law on which these calculations are based dC 11 dz h wherein C is the mass concentration, v is the velocity of sedimentation calculated from Stokes law, 1 is the time and h is the height of the chamber. By combining the integrated form of this equation with Stokes formula the following expresison for the average diameter d is obtained:

wherein 1; is the coeificient of viscosity of the gaseous medium, h is the height of the chamber, C and C are the average mass concentrations of the solid in the gaseous medium a and b minutes, respectively, after the burst, g is the acceleration of the solid body due to gravitation, p is the density of the solid material and t is the time.

Using the values 07 (viscosity of air)=1.86 10- poises, h:250 cm., g (acceleration) :981 cm. per second, the following expression for the average diameter D in microns is obtained:

where Q, is the average mass concentration of solid in air during the period one to five minutes after the burst, C is the average mass concentration of solid in air for the period eleven to fifteen minutes after the burst, and p is the density of the solid.

Example III Ten grams of zinc oxide having an average particle size of about 0.2 micron diameter, which is available commercially as a pigment grade of zinc oxide, 50 grams of water and 0.1 gram of propylene glycol monooleate, are passed through a colloid mill until a uniform slurry is obtained. The resulting surface-treated zinc oxide is filtered from the water, dried at C. and passed through a Mikro-Pulverizer to disintegrate the filter cake into individual solid particles. A cylindrical cellophane container is charged with 2.83 grams of this surfacetreated zinc oxide and a number 6 blasting cap inserted in an axial position in the container. When the cap is fired, a zinc oxide aerosol is obtained in which an average (based on results of three concordant experiments) of 77%, 29% and 15% by weight of the particles remain suspended for one to five, six to ten and eleven to fifteen minutes, respectively, after the explosion. A control aerosol prepared by the same method from untreated zinc oxide of the same particle size contains only 36%, 16% and 12% by weight of air-borne particles during the same respective periods of time.

As hereinbefore stated, my process comprises the fol lowing steps: (1) comminuting a solid material until it has a particle size of less than 20 microns weight median diameter, (2) coating the surface of the resultant particles with a surface-active agent which reduces cohesion between said particles, and (3) subjecting the resultant surface-treated finely-divided solid material to an explosive burst. Aforementioned steps (1), (2) and (3) may be carried out separately in the order named or steps (1) and (2) may be effected simultaneously as illustrated in Examples I and II.

While this invention has been illustrated with particular reference to the preparation of aerosols of egg albumin, zinc oxide and lithopone, it is to be understood that said invention is generic to all solid materials which are capable of being reduced to a particle size of less than 20 microns weight median diameter. Included among aerosols which may be had in accordance with this invention are: those of cornstarch; yeast; proteins, e.g., blood albumin, egg albumin, gliadin, zein and casein; organic and inorganic insecticides, e.g., pyrethrum, rotenone, ferric dimethyl dithiocarbamate, p,p'-dichlorodiphenyltrichloroethane, dodecyl thiocyanate, lead arsenate, calcium arsenate, sulfur, basic copper carbonate, barium fiuosilicate and calcium fiuosilicate; white pigments and pigment extenders, e.g., zinc oxide, white lead, lithopone, barium carbonate, barium sulfate, clay, calcium carbonate and aluminum silicates; colored pigments, e.g., chrome yellow, basic zinc chromates, phthalocyanine blue, cadmium yellow, red iron oxide, carbon black and the like. The novel process of this invention is particularly well adapted for the provision of aerosols of insecticides and proteins.

It is to be understood that the finely divided solid material dispersed in accordance with this invention should have a particle size of less than 20 microns weight median diameter. Optimum results are obtained when said material has a particle size of less than 10 microns weight median diameter. By weight median diameter, as employed herein and in the appended claims, it is meant that the amount by weight of the solid material particles having a diameter less than the specified diameter equals the amount by weight of said particles having a diameter greater than said specified diameter. Comminution of the solid material to the required particle size can be effected by any of the several milling procedures known to the prior art, e.g., by pulverizing it in a ball mill, in an impact mill such as a Mikro-Pulverizer, in a jet pulverizer or high speed gas disintegrater such as a Micronizer, or by a combination of any of these methods.

As hereinbefore stated, surface treatment of the finely divided solid material can be effected during or subsequent to the aforesaid comminuting operation. Surface treatment during the comminuting operation is preferred because it is more economical. The surface-active agent may be admixed in the dry state with the solid material or it may be dissolved or suspended in a liquid medium. When the latter procedure is followed, the solution or dispersion of the surface-active agent is admixed with the solid material, the liquid medium being thereafter re moved from said material by filtering and/ or drying the same.

While this invention has been illustrated with particular reference to the use of diethylene glycol monolaurate and propylene glycol monooleate as surface-active agents, it is to be understood that said invention is generic to the use of polyhydric alcohol partial esters of unsubstituted long chain fatty acids, i.e., of an unsubstituted fatty acid containing at least 8 carbon atoms, such as diethylene glycol monolaurate, propylene glycol monooleate, gylcerol monolaurate, diethylene glycol monostearate and mannitan monolaurate. The alcohol groups of the polyhydric alcohol not esterified by the long chain fatty acid may be esterified with other acids, organic or inorganic, e.g., phosphoric acid, as in soya lecithin. Diethyleneglycol monolaurate provides optimum dispersion.

The amount of surface-active agent employed may be varied within relatively wide limits. However, said surface-active agent is ordinarily applied in an amount within the range of from 0.1% to 5% of the weight of the finely divided solid material being treated. Optimum results are usually attained when said surface treating agent is employed in an amount within the range of from 0.1% to 2% of the weight of the finely divided solid material.

In the third, or dispersal, step of this invention any type of explosive burst may be employed to generate the aerosol. The palrticular type of explosive burst selected will be dependent upon the particular type of solid being dispersed. When a solid which is sensitive to heat is being dispersed, it is preferred to use an endothermic bursting charge, such as that described in Example II. Said endothermic bursting is broadly elfected by rupturing in the air a receptacle containing the finely divided surface-treated solid material and a fluid which expands endothermally, said fluid having a vapor pressure of at least three atmospheres at 25 C., being inert toward, and a non-solvent for, said solid material and being under a pressure of at least three atmospheres in said receptacle. When a solid which is not sensitive to heat is being dispersed, any type of explosive burst can be employed. For example, the finely-divided surface-treated solid can be enclosed with an ordinary explosive in a container and the explosive detonated. In another example, a missile such as a grenade or a shell containing a finely-divided surface-treated solid and an explosive can be projected into the air by suitable apparatus, e.g., a rifle or a mortar, and exploded by customary means in the locality where the aerosol is desired.

The process of this invention is of particular utility for the dispersal of proteins and solid insecticides or other agricultural poisons in an unusually effective form.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

Having described the present invention, the following is claimed as new and useful:

1. In a process of obtaining an aerosol, the step which comprises bursting in the air a closed frangible receptacle containing carbon dioxide under a pressure of at least 3 atmospheres and a solid material that has a particle size of less than 20 microns weight median diameter and is surface-coated with from 0.1% to 5% by weight of a polyhydric alcohol partial ester of an unsubstituted longchain fatty acid.

2. In a process for obtaining an aerosol, the step which comprises bursting in the air a closed frangible receptacle containing carbon dioxide under a pressure of at least 20 atmospheres and a solid material that has a particle size of less than 10 microns weight median diameter and is surface coated with from 0.1% to 2% by weight of a polyhydric alcohol partial esters of an unsubstituted longchain fatty acid.

3. The step in a process of obtaining an aerosol according to claim 2 wherein. the solid material comprises an insecticide.

4. The step in a process of obtaining an aerosol according to claim 2 wherein the solid material comprises a pigment.

5. The step in a process of obtaining an aerosol according to claim 2 wherein the solid material comprises a protein.

6. In a process for obtaining an aerosol, the step which comprises bursting in the air a closed, frangible receptacle containing carbon dioxide under a pressure of at least -20 atmospheres and a solid material which has a particle size of less than 10 microns weight median diameter and which is surface coated with from 0.1% to 2% by weight of diethylene glycol monolaurate.

References Cited in the file of this patent UNITED STATES PATENTS 1,433,732 Lewis et a1. Oct. 31, 1922 1,958,700 Harris May 15, 1934 2,028,217 Hufiman Jan. 21, 1936 2,140,375 Allen et al. Dec. 13, 1938 2,321,023 Goodhue et al. June 8, 1943 2,345,891 Sullivan et al. Apr. 14, 1944 2,358,986 McGovran Sept. 26, 1944 2,364,145 Huppke et a1 Dec. 5, 1944 OTHER REFERENCES Science News Letter, pages 117-118, February 19, 1944.

Science News Letter, page 57, column 3, July 22, 1944.

Journal of Economic Entomology, volume 37, No, 1, page 136, column 2. 

1. IN A PROCESS OF OBTAINING AN AEROSOL, THE STEP WHICH COMPRISES BURSTING IN THE AIR A CLOSED FRANGIBLE RECEPTACLE CONTAINING CARBON DIOXIDE UNDER A PRESSURE OF AT LEAST 3 ATMOSPHERES AND A SOLID MATERIAL THAN HAS A PARTICLE SIZE OF LESS THAN 20 MICRONS WEIGHT MEDIAN DIAMETER AND IS SURFACE-COATED WITH FROM 0.1% TO 5% BY WEIGHT OF A POLYHYDRIC ALCOHOL PARTIAL ESTER OF AN UNSUBSTITUTED LONGCHAIN FATTY ACID. 